Hi Sentmoraap,
all development is welcome
the best place for development is generally the mailing list.
What I would recommend as a first step is to checkout SuperTuxKart from our Subversion repositories at sourceforge. You can then peek around at the code a bit. When you picked an area you would like to work on, or if you would like ideas, you can then post on the mailing list (or here, though the ml is preferred) so we can discuss about it a bit.
The next step is to code
Then you can use SVN to generate patches describing your changes, and you can then send them to us.
Good luck, and welcome aboard shall you decide to dive in deeper!